sweeney



Sept. 10, 1963 H. E. swEENEY STEREOPHONIC RECEIVING METHOD AND APPARATUS 3 Sheets-Sheet 1 Filed April 13, 1959 ww R r n mem NS N ww Ns. L. E .m d, I Y mBf 0 wwnesszs Sept. 10, 1963 H. E. swEENl-:Y

sTEREoPHoNIc RECEIVING METHOD AND APPARATUS 3 Sheets-Sheet 2 Filed April 13, 1959 N .mf-A

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Sept. 10, 1963 H. E. swl-:ENEY 3,103,555

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IF Carrier Frequency UnitedStates Patent O 3,103,555 STEREOPHNIC RECEIVING METHOD AND APPARATUS The present invention relates to methods and Iapparatus for reception of stereophonic radio signals and more particularly to methods and apparatus for receivingy signals in which a compatible monophonic component issupplied as conventional amplitude modulation and stereophonic intelligence is supplied as angle modulation of the saine carrier wave.

When sound is transmitted -by the ordinary system over radio channels, only a single channel is normal-ly provided for monophonic reception and audio perspective is entirely lost since the spatial displacement between the sounds-received by the two ears of |the listener bears no relation to the spatial displacement of the sounds at the microphone which feeds the transmitter. Stereophonic reception has heretofore been demonstrated using two microphones, set up at locations on each side 'of a stage on which an orchestra is situated. Each microphone is connected by a separate radio channel to one of two loudspeakers placed similarly as t-he microphones but in a listening chamber where the receiving apparatus is situated. By such an arrangement an auditory etfect'may :be obtained which is substantially the same ias-though the orchestra or other source of sound were actually,loca-ted in front `of ,the listener. rather than the sound being reproduced by the loudspeakers.

In yapplying stereophonic sou-nd to radio channels there arises one very serious obstacle from a practical point of view. The need for two separate channelsfor the transmission of `a single program has heretofore prohibitedv ing to .a minimum the additional investment required atv the transmitter as Well as the additional investment required of prospectivelisteners. f

yOne proposed stereophonic system using a. single chaunel has been described in detail in Electronics malga-v zine, issue of February 1941,at pages 34 to 36. That proposed system suggests transmission of the audio signal from a iirst microphone as amplitude modulation and` that from a second microphone as `frequency modulationof Ithe same carrier. Such a system has the disadvantage that a conventional receiver will reproducev the. signals from fthe first microphone only. Thus a conventicnalre-Y ceiver produces sound signals corresponding .to those. heard at one end of a stagev onwhichthe orchestra is' located. A primary requisite of a genuinely compatible system is that a conventional receiver should produce balanced monophonic sound `substantially corresponding.

to the soundV effects which would be heard by. alistener seated near the center of the auditorium. in which the orchestra is located.

In another previously proposed stereophonic transmission system audio. signals A land B from spaced micro-l phones are transmittedA by adding the signals and transmitting the resulting sum signal A-l-B as yfrequency modulation `of la VHF carrier wave, and simultaneously sulbtracting the signals and transmitting the `difference signal.

ice

. asinodulation of a super. audible. subcarrierin the same channel. At the receiver the two. signals, after processing `by different circuits tuned respectively to the canrierwave and to the subcarrier wave are ldemoduiated ,and then (ilr) added to produce the rst audio signal A, and (2) algebraically subtracted to produce the second audio sig-- simultaneously accommodate the main FM carrier and. It is not suitable ttor use inA the AM broadcast bandwhere bandwidth maximum isthe supersonic subcarrier.

restricted by. FCC regulations to ten kilocycles; .Such la system has been proposed for use in the FM broadcastV band and hence: would: provide mcnophonic reception only to those listeners having FM band receiving appa-.

In addition, such a system'requires circuitry of ratus'.

special design and considerable expense 'for stereo-phonic reception using the suboanrier signal.

Accordingly, a primary `object of the present invention is to provide compatible reception of monophonic sound for listeners having conventional AM broadcast receivers and -to simultaneously provide reception of stereophonic soundV for listeners having a pair of similarl broadcast band receiving sets.

It is a further object of the present invention to en-` able stereophonic 'reception by Combined utilization of two conventional AM broadcast receivers tuned to the'V same broadcast band transmission.

lIt 'is `arr-additional object of. the prescntinvention to provide a dual channel stereophonc receiver requiringno explicit maitrixing at the receiver outputs..

It' iswanother objectV of the present invention to provide a stereophonic receiving apparatus including -a piairi of loudspeakerswhich apparatus Imay be alternatively` adjusted for production of `balanced monophonic sound in;

Concert `from both loudspeakers.

The receivingapparatus of-thepresent invention iinds.l

particular application in reception of'signals transmitted in accordance with a-system yfor. compatible stereophonic transmission: as describedy in detailrin the copending applicationzof Harold E. Sweeney and CharlesW. Baugh, Ir. Serial No. 808,038; led April 22, 1959, now Patent No..3,069,679,- issued December 18, 1962 and-assigned to the. same `assignee as the present applica-tion. In that systemamplitude modulation of a broadcast band carrier wave is used for transmission of the algebraic sum A-l-B of. the twostereo signals and frequency modulation of the same carrier wave is used for transmission:ofthe difference signal A-B. The transmission system there utilized enables a conventional AMl receiver tuned to the: transmitted carrier to reproduce the sum signal A+B which signal contains. substantially equal components of the. signal from each microphone andtherefore will-producera balanced monophcnic sound; The aforementioned copending application proposes -aspecial receiver'for use with the transmission systemfwhich receiver wouldhave, two separate detectors `one being an'amplitude modulation detector. and the other being an FM detector so that .the-A+B. audio signal andthe- A-B- audiosignal {ceivingchannels which channels are adjusted'V diiierentlv relative to a received carrier center frequency so that one channel will produce the rst audio signal A and the other channel will produce the second audio signal B. More specifically, a rst receiver channel is adjusted so that the received carrier is tuned to the positive slope of the rst receiver frequency response characteristic. The other receiver channel is tuned so that the received carrier signal lies an equal distance down on the negative slope of the second receiver frequency response characteristic. Each receiver will detect and reproduce the monophonic component A+B. In `addition the-first receiver will slope detect the frequency modulation signal to produce a stereophonic difference signal component A-B and will algebraically add the sum signal A+B and the difference signal A-JB to produce a signal corresponding to the coherent sound signal A from Vthe first microphone. Similarly the second receiving channel will slope detect the frequency modulation signal and will algebraically subtract the demdulated difference signal A-B from the demodulated sum signal A+B to produce a coherent sound signal corresponding to the signal B of the second microphone.

The foregoing and other objects and features of the present invention will be more clearly apparent from the following description taken with the accompanying drawing, throughout which like reference characters indicate like parts, which drawing forms a part of this application and in which:

FIGURE l Iis a block schematic diagram showing a stereophonic sound transmission system for broadcasting signals of the type utilized by the receiving systems of this invention.

lFIGURE 2 is a block schematic diagram showing a binaural receiving system in accordance with the present invention.

FIGURE 3 is a block schematic diagram of a second embodiment of a receiving system in accordance with the present invention, and

FIGURE 4 is a plurality of curves useful in explaining the operation of the present invention.

Referring to FIGURE l of the drawing, the system there shown includes a standard AM broadcast transmitter including a master oscillator 20, a class C amplier 24, and an amplitude modulator 26 for applying signals to a transmitting antenna 2S. The conventional broadcast transmitter is modified by addition of a frequency modulator stage 22, which modulates the output of oscillator 2.0 in accordance with a stereophonic difference signal A-B which is applied to the FM modulator by means lof the audio signal conductors 19. Thus ythe system ltrans-mits a standard monophonic audio signal component A+B as amplitude modulation of the transmitted carrier signal and in addition multiplexes on the carrier an FM signal corresponding to the stereophonic difference signalA-B.

Microphones A and B are spaced apart on the stage of a studio on which is located an extended sound source such las an orchestra engaged in the performance of a musical composition. The output of microphone B is amplied by an laudio preamplifier 11 and applied to a primary winding of an audio transformer 13 which has a'secondary winding comprising two winding portions 1S and 16 joined at a center tap. The sound signals from microphone A `are applied through an audio preamplifier 12 to the primary winding of a transformer 14 having a secondary winding 17. The secondary Winding 17 is connected serially with the winding portion 15 to the conductors 18 for applying signals to the amplitude modulator Z6. Similarly, the transformer winding 17 is connected serially with the winding portion 16 to the audio conductors 19 which apply signals to the FM modulator 22. The combining network comprising transformers 13 and 14 is an arrangement which is known per se for providing a sum signal A+B at conductors 18 and a difference signal A-B at conductors 19. By

inspection ofthe connections of the secondary windings of transformers 13 and 14 it can be seen that the windings 16 and 17 provide subtraction of the audio signals A and B and apply the diiference signal to FM modulator 22. Windings 15 and 17 are connected in additive polarity so that the sum signal A+B is provided at audio conductors 18 and is .applied to the amplitude modulator 26.

One transmission system substantially similar to FIG- URE l is described in detail in the aforementioned copending application of H. E. Sweeney and C. W. Baugh, Ir. Accordingly, it will sulce for the-purposes of the present application to state that a system such as that of FIG. l provides a composite signal including a carrier wave which is amplitude modulated and frequency modulated respectively, with a sum signal A+B and a difference signal A-B wherein A and B are coherent audio signals respectively corresponding to sound intelligence at the spaced locations of the microphones A and B.

In ll-TIG. 2 there is shown `one embodiment of a method and apparatus for receiving signals of the type produced by a transmission system such as that of FIG. l. FIG. 2 shows a pair of radio receivers for receiving signals in the AM broadcast band. The receivers of FIG. 2 are conventional in many respects and particularly in that they respectively include receiving antennas 30 and 40, conventional `heterodyne mixer circuits 32 and 42, conventional local oscillators 34 and 44, conventional AM detector circuits 37 and 47, conventional audio amplifiers 38 and 48, and conventional loudspeakers 39 and 49. ln accordance with the present invention, the loudspeakers 39 and 49 are spaced apart a predetermined distance in a listening chamber or auditorium. The rst receiver of FIG. 2 includes an intermediate frequency amplier and bandpass or high-pass lter 36 having a frequency response characteristic substantially as shown by curve 35 in FIG. 2. Similarly, the second receiver of FIG. 2 includes an intermediate frequency amplifier and bandpass or low-pass lter 46 having a frequency response characteristic substantially as shown by curve 45.

In utilizing fthe receivers of FIG. 2 in accordance with the present invention, the two receivers are tuned to the same AM broadcast channel. The local oscillator 34 of the `first receiver is preferably adjusted to produce a lower frequency output than that of local oscillator 44. Mixer 32 accordingly produces a modulated intermediate frequency carrier signal having a predetermined center frequency fc which lies on the positively sloping portion of the frequency response characteristic curve 35. The intermediate frequency carrier signal will of course have Ithe same frequency modulation and amplitude modulation as that of the incoming signal as received by antenna 30. Accordingly, the intermediate frequency carrier will vary in frequency on both sides of the center frequency fc between the deviation limits f1 and f2 as shown in FIG. 2. Also the IF carrier will vary in amplitude in accordance with the sum signal A+B. When the so modulated intermediate frequency carrier is applied to the sloping portion of the bandpass curve '35, the IF amplifier 36 will amplify signals at frequency f2 to a greater degree than signals at frequency f1. Accordingly, IF amplifier 36 operates to convert the frequency modulation to an amplitude modulation component. That is, in response to the A-B frequency modulation the IF amplifier 36 generates an amplitude modulation component which varies in direct proportion to the instantaneous frequency of the intermediate frequency carrier as determined by the frequency modulation thereof.

The so `generated amplitude modulation component, which corresponds to the stereophonic dierence signal A-B, is additively combined with the pre-existing amplitude modulation of the intermediate frequency carrier, which pre-existing amplitude modulation corresponds to the stereophonic sum signal A+B. Accordingly, the

amplitude modulation of the intermediate frequency carrier as applied to the AM detector 37 has an envelope which varies substantially in accordance with the algebraic sum of the sum signal A--l-B and the difference signal A-B. The algebraic sum corresponds to the signal A and is substantially.representative of fthe sound intelligence impinging upon the first microphone A of the transmission system as shown in IFIG. l. The audio signal A from detector 37 is amplified in the usual man-V ner by audio amplifier 38 and is applied to the first loudspeaker 39.

The second receiver of FIG. 2 operates in substantially the same manner in general as the first receiver of FIG. 2. The essential difference between the two receivers is that the local oscillator 44 of the second receiver is tuned higher than normal in frequency so that it beats with the incoming radio frequency carrier as received by the antenna 40 to produce an intermediate frequency carrier Wave having a center frequency fc which is higher in frequency than the maximum response portion of the IF frequency response characteristic as shown by the curve 45.

The intermediate frequency carrier wave as applied to the IF amplifier 46 will have a center frequency fc as shown in conjunction with the curve 45, and will vary in frequency between the limits f3 and f4, with the variations in frequency corresponding to the frequency modulation of the incoming carrier. When the stereophonic difference signal A-B has a maximum positive value the intermediate frequency carrier Iwill approach the limit frequency f4. Likewise, when the stereophonic difference signal A-B has a maximum negative value the carrier will approach the limit frequency f3. When the sound signals received by the microphones A and B are identical both in amplitude and in phase, the difference signal A-B will have a zero value. In that instance the frequency of the intermediate frequency carrier as applied to IF amplifier 46 will have a frequency value corresponding to the center frequency fc as shown in conjunction with curve 45t. When the stereophonic difference signal has a negativev value, the modulated carrier will approach the frequency f3 and the IF amplifier 46 will have a maximum amplification factor. When the stereophonic difference signal has a maximum positive value, the carrier will be located at f4 and IF amplifier 45 will have a minimum amplication factor. Accordingly the IF amplifier 46 generates an amplitude modulation component ywhich varies in inverse proportion to the instantaneous frequency of the intermediate frequency carrier and, therefore, in inverse proportion to the amplitude value of the stereophonic difference signal A-B. Thus the amplitude modulation component generated by the amplifier 46 responding to the frequency modulation has the form (A -B). That inverse function amplitude modulation component is combined with the pre-existing amplitude modulation of the IF carrier as received by the antenna 40'. Accordingly, the envelope amplitude of the output signal from I-F amplifier 46' varies in accordance with the algebraic difference of the sum signal (A+B) and the stereophonic difference signal (A -B), and is substantially representative of the audio signal as produced by the microphone B of FIG. l. The composite amplitude modulation of the intermediate frequency carrier fciAf is detected by the conventional AM detector 47 and the audio signal B thus produced is applied through-the conventional audioy amplifier 48 to the second loudspeaker 49.

In FIG. 3 there is shown a second embodiment of receiving apparatus employing the receiving method of the present invention. The antenna 50, mixer circuit 52, local oscillator 54 and the intermediate frequency amplifier 55 preferably are conventional circuits such as commonly used in AM broadcast receivers. The output of intermediate frequency amplifier 5-5 is coupled to a first channel comprising a bandpass filter 57 which is coupled to a conventional amplitude modulation detector 60. The output terminals of the first AM detector 60' are connected to a conventional audio amplifier 64 which applies signals 4to a conventional loudspeaker 68; The output of intermediate frequency amplifier 55 is further coupled to a second channel comprising a bandpass lter 59 having its output terminals connected to the input circuit of a second amplitude modulation detector 62. The output terminals of detector 62 are connectedy to the input circuit of an audio amplifier 66 which applies signals to a conventional loudspeaker 69*l The loudspeakers 68 and 6-9 are preferably spaced apart in a listening chamber or auditorium in the same manner as heretofore described in connection with the apparatus of FIG. 2.

Intermediate frequency amplifier 55 preferably has a sufiiciently wide bandpass to provide linear amplification of the amplitude modulation side bands and the frequency modulation side bands of the intermediate frequency carrier signal provided by the mixer circuit SZ when the receiver is tuned to receive an AM--FM multiplex signal of the general type transmitted by the apparatus of FIG. 1 or the apparatus of -aforementioned application S.N. 808,038. The intermediate frequency carrier has a center frequency fc" about which it is frequency modulated to the deviation limits f5 and f6 as shown in FIG. 4. The center frequency fc may for example be 456` kilocycles as utilized in conventional AM receivers. The bandpass filter 57 of FIG. 3 has a frequency response characteristic as shown by curve '70 in FIG. 4. The geometrical center or maximum response portion of the bandpass characteristic 7)1 lies approximately 3.0-4.0 kc. above the center frequency fc" of the intermediate frequency carrier. For example, the center of the maximum response portion of bandpass characteristic 70 may be approximately 460 kilocycles so that the center frequency fc" lies on the positively sloping portion of the characteristic curve 70. The bandpass filter 57 may comprise any network in which for constant amplitude and phase of the input signal the amplitude of the output signal becomes substantially lineanly lvariable with frequency. That is, the slope of the bandpass characteristic 70t between fthe frequencies f5 and f6 as shown in FIG. 4 preferably should be substantially linear and the phase shift characteristic of the bandpassfilter 57 preferably should be substantially linear between those limits. It has been found in practical receivers, that appreciable non-linearity cannot be tolerated without deteriorating the stereophonic effect produced by speakers 68 and 69.

The bandpass filter 59 of FIG. 3 is substantially similar to the bandpass filter 57 with the exception thatit has a bandpass characteristic substantially as shown by curve 72 of FIG. 4. The bandpass characteristic 72 has a negatively sloping portion which is substantially linear between the limits of frequency modulation f5 and f6 as shown in FIG. 4. It will be appreciated that the fbandpass filter 59 may comprise any resonant circuit or amplifier or filter which is so tuned or aligned that the negatively sloping portion of the frequency response characteristic curve 72 coincides with the frequency bandv (fciAf) occupied by the frequency modulated intermediate frequency carrier signal. teristic of the bandpass filter 59 for the purpose of the present invention is that it provides, in response to a. frequency modulated input signal of constant amplitude.,4

An essential charac-- for the purpose of analysis of the system to consider the constant amplitude sine wave '74 as shown in `HG. 4. Such a constant amplitude frequency modulated signal would occur, for example, when audio signas A and B are equal in amplitude and opposite in phase so that A+B equals Zero and A+B=2A. Curve 7o of FIG. 4 indicates the manner in which the frequency of the sine wave 74 varies in drequency as a function of tirne. At a time to to the sine Wave 74 has a frequency fc, for example 456 kilocycles. This condition corresponds to a time at which the stereophoriic difference signal A+B has a zero value. At a Ilater time r1 .the `frequency of the sine Wave 74 has increased to a maximum corresponding to the frequency f6. This con-dition indicates a maximum amplitude value of the stereophcnic difference signal A+B. At a still later time, t3, the frequency of the sine Wave 74 has decreased, through the mean frequency fc", to a minimum frequency value f5 as indicated by the minimum of curve 7d. This condition corresponds to a maximum negative value of the stereophonic difference sig-nal A+B.

Projection of the curve 7d to the sloping portion of the bandpass characteristic 7d of the bandpass filter 57 illustraites that #as the intermediate frequency sine wave 74 decreases in frequency between the times t1 and z3 it moves down ion the sloping portion of the bandpass characteristic 7d and is thereby progressively more attenuated. Thus bandpass filter 57 inodines the amplitude of the intermediate frequency carrier Wave as a direct function of the frequency modulation thereof. The amplitude modulation component generated by the filter E57 is illustrated by curve 78 in IFIGURE, 4. Curve 7S further indicates the manner in which the output of amplitude modulation :detector 6d would vary if a constant amplitude frequency modulated carrier Wave such as that shown at 74 were applied to the input circuit of bandpass filter 57. When the lfrequency modulation of the sine Wave '74 is considered as representing the stereophonic difference signal A+B, then it is seen .that the curve '78 at the output yof idetector ylili yrepresents the audio frequency component resulting Afrom slope `demodulation of the stereophcnic :difference signal. Iif it be assumed that the rfr-equency modulated Wave 74 carries no amplitude modulation, then the laudio output wave 7S from detector 60 would vary as a direct function of the sterecphonic difference signal.

Considering the projection of curve '76 to the negatively sloping portion of the bandpass characteristic '72. of the bandpass filter 59, it may be readily seen that as the IF carrier frequency decreases between the times t1 and t3 the attenuation provided by filter 59 pnogressively decreases. rPhi-us the amplitude of ,the intermediate frequency carrier transmitted by filter 59 will progressively increase between the times t1 and i3 as shown by the curve '79 in FIG. 4. The amplitude modulation component thus generated by the filter 59 is detected by the amplitude m-odulation detector 62 and is applied through the audio amplifier o6 to the loudspeaker o9. The audio signal component resulting from the action of fiiter 59 on frequency modulated wave 74 is illustrated by the curve 79 in FIG. 4. It is important to note that curve 79 corresponds inversely to the curve 78, thus if the curve 7 3 represents a positive or direct function of the stereopfhonic difference signal A+B, then curve 79 represents a negative or inverse function of the stereophonic diiference signal.

When the receiver of FlG. 3 is tuned to a transmission of the type radiated by the apparatus of FlGURE l, the intermediate `frequency carrier as translated by intermediate frequency amplifier 55 will carry amplitude modulati-on corresponding to the sum signal A+B. lFilter 57 adds to that pre-existing A+B amplitude modulation, a second amplitude modulation component corresponding directly to the stereiophonic difference signal A+B. Accordingly the modified amplitude modulation envelope as applied Ito the detector 6d varies substantially in accordance with the algebraic sum of the sum signal (A+B) and the stereophonic difference signal (A+B). Thus the audio output signal yfrom the amplitude modulation derector o@ will be representative of the audio signal A as produced by the first microphone olf the apparatus of Fifi. l.

Conversely ythe til-ter 59 creates an `amplitude modulation component which varies as an inverse ifuinction of the stereophonic difference signal A+B. The amplitude modulation created by filter 59 adds to the pre-existing sum signal amplitude modulation as translated by IF ampli ier 55, and the envelope amplitude of the intermediate frequency carrier at the output of iilter 59 varies in accordance with the algebraic difference of the sum signal A+B and the stereophonic difference signal A+B. Thus the envelope amplitude of the carrier signal as applied to the detector varies substantially as a function of the audio frequency signal from the second microphone B in the lapparatus of yFIG. `l. Accordingly the spaced loudspeakers 63 and 69 produce stereophonic audio signals A and B respectively.

yit will be apparent that the apparatus of FIG. 3 is compatabile for reception of either inonophonic ior stereophonic signals transmitted in the AM broadcast band. That is, the receiver of FIG. 3 may be tuned to any ccnventional AM broadcast station and will receive conventional monophonic AM signals [to produce conventional monophonic sound in each of the speakers 68 and 69 simultaneously with the produced sound being in phase and mutually compatible.

While the present invention has been shown and described in certain preferred embodiments only, it will be understood by persons skilled in the art that it is not so limited, but is susceptible of various changes and modifications within the spirit and scope thereof.

l claim as my invention:

l. A stereophonic receiving system for receiving a radio Wave having amplitude modulations representative of A+B, and frequency modulations representative of A+B, Iwherein A and B are coherent audio signals respectively corresponding to sound intelligence at first and second spaced locations, first and second receiving channels, means applying said amplitude and said frequency modulations to said firs-t and second receiving channels, said `lirst receiving channel having a positively sloping frequency response characteristic over the range of frequency deviations of said frequency modulations to modify the amplitude of said amplitude modulations, representative of A+B, in direct proportion to the instantaneous frequency of said frequency modulations, representative of A+B, to produce a first radio frequency signal having envelope amplitudes corresponding to signal A, said second receiving channel having a negatively sloping frequency response characteristic over the range of frequency deviation of said frequency modulations to modify the amplitude of said amplitude modulations, representative of A+B, in inverse proportion to the instantaneous frequency of said frequency modulations, representative of A+B, to produce a second radio frequency signal having envelope amplitudes corresponding to signal B, said rst channel including first detector means for demodulating said first radio frequency signal, and said second channel including second detector means to demodulate said second radio frequency signal.

2. A stereophoriic receiving system as described in claim l wherein said means applying said amplitude and frequency modulations to said first and second receiving channels includes heterodyne converter means responsive to said radio Wave for producing an intermediate frequency carrier signal having a predetermined center frequency and having said amplitude and frequency modulations thereon, said first channel having a positively sloping frequency response characteristic at said center frequency and over the range of frequency deviations of 9 said frequency modulations, said second channel having a negatively sloping frequency response characteristic at said center frequency and over the range of frequency deviations of said frequency modulations.

3. A stereophonic receiving system for receiving a radio wave having amplitude modulations representative of A+B and frequency modulations representative of A-B wherein A and B are coherent audio signals respectively corresponding to sound intelligence at first and second spaced locations, first and second superheterodyne receivers, respectively including first and second heterodyne converter means responsive -to said radio wave for respectively producing first and second intermediate f requency carrier :signals having respectively first and second center frequencies with said amplitude modulations representative of A+B, and said frequency modulation representative of A+B, thereon, a first intermediate frequency amplifier coupled to said rst heterodyne converter means and having a positively sloping frequency response characteristic at said first center frequency and over the range of frequency deviations of said frequency (modulations thereon to modify the amplitude of said amplitude modulations, representative of A+B, in direct proportion to the instantaneous frequency of said frequencymodulations, representative of A-B, to produce a first radio frequency signal having envelope amplitudes corresponding to signal A, first detector means for demod' ulating said first radio frequency signal and said sound reproducing means coupled to said first detector means for reproducing signal A, second intermediate frequency amplifier coupled to said second heterodyne converter means and having a negatively sloping frequency response characteristic at said second center frequency and over the range of frequency deviations of said frequencies modulations thereon to modify the amplitude of said amplitude modulations, representative of A+B in inverse proportion to the instantaneous frequency of said frequency modulations, representative of A-B, to produce a second radio frequency signal having envelope amplitudes corresponding to signal B, second detector means coupled to said second intermediate frequency amplifier for demodulating said second radio frequency signal and sound reproducing means ooupled to said second detector means for reproducing signal B.

4. A stereophonic receiving system for receiving a radio wave having amplitude modulations representative of A+B and frequency modulations representative of Ae-B wherein A and B are coherent audio signals respectively corresponding to sound intelligence at first and second spaced locations, said receiving system comprising; first and second superheterodyne receivers, respectively including first and second heterodyne converter means responsive to said radio wave for respectively producing first and second intermediate frequency carrier signals having respectively first and second predetermined center frequencies with said amplitude modulations representative of A+B, and said frequency modulations representative of A-B, thereon, said first receivers including a first intermediate frequency amplifier coupled to said first heterodyne converter means, said first intermediate frequency amplifier having a positively sloping frequency response characteristic at said first center frequency and over the range of frequency deviations of said frequency modulations, to modify the amplitude of said amplitude modulations, representative of A+B, in direct proportion to the instantaneous frequency of said frequency modulations, representative of A-B, to produce a first radio frequency signal having envelope amplitude corresponding to signal A, a second intenmediate frequency amplifier coupled to said second heterodyne converter means and having a negatively sloping frequency response characteristic at said second center frequency and over the range of frequency deviations lof said frequency modulations to modify the amplitude of said amplitude modulations, representative of A+B, in inverse proportion to the instantaneous frequency of said frequency modulations, representative of A-B, to produce a second radio frequency signal having amplitudes corresponding to signal B, a first amplitude coupled to said first intermediate frequency amplifier to demodulate said first radiov frequency signal, lirst sound reproducing means coupled to said first detector for reproducing signal A, a second amplitude detector coupled to said second intermediate frequency ampliiier to demodulate said second radio frequency signal and second sound reproducing means coupled to said seco-nd detector to reproduce signal B.

References Cited in the file of this patent UNITED STATES PATENTS 2,378,298 Hilferty June 12, 1945 2,512,530 OBrien et al June 20, 1950 2,562,703 Dome July 31, 1951 y2,633,529 Eltgroth Mar. 31, 1953 2,654,885v rWilmotte Oct. 6, 1953 2,874,221 Dauguet Feb. 17, 1959 2,878,319 Leek Mar. 17, 1959 FOREIGN PATENTS 120,269 Australia Aug. 23, 1945 

1. A STEROPHONIC RECEIVING SYSTEM FOR RECEIVING A RADIO WAVE HAVING AMPLITUDE MODULATIONS REPRESENTATIVE OF A+B, AND FREQUENCY MODULATIONS REPRESENTATIVE OF A-B, WHEREIN A AND B ARE COHERENT AUDIO SIGNALS RESPECTIVELY CORRESPONDING TO SOUND INTELLIGENCE AT FIRST AND SECOND SPACED LOCATIONS, FIRST AND SECOND RECEIVING CHANNELS, MEANS APPLYING SAID AMPLITUDE AND SAID FREQUENCY MODULATIONS TO SAID FIRST AND SECOND RECEIVING CHANNELS, SAID FIRST RECEIVING CHANNEL HAVING A POSITIVELY SLOPING FREQUENCY RESPONSE CHARACTERISTIC OVER THE RANGE OF FREQUENCY DEVIATIONS OF SAID FREQUENCY MODULATIONS TO MODIFY THE AMPLITUDE OF SAID AMPLITUDE MODULATIONS, REPRESENTATIVE OF A+B, IN DIRECT PROPORTION TO THE INSTANTANEOUS FREQUENCY OF SAID FREQUENCY MODULATIONS, REPRESENTATIVE OF A-B, TO PRODUCE A FIRST RADIO FREQUENCY SIGNAL HAVING ENVELOPE AMPLITUDES CORRESPONDING TO SIGNAL A, SAID SECOND RECEIVING CHANNEL HAVING A NEGATIVELY SLOPING FREQUENCY RESPONSE CHARACTERISTIC OVER THE RANGE OF FREQUENCY DEVIATION OF SAID FREQUENCY MODULATIONS TO MODIFY THE AMPLITUDE OF SAID AMPLITUDE MODULATIONS, REPRESENTATIVE OF A+B, IN INVERSE PROPORTION TO THE INSTANTANEOUS FREQUENCY OF SAID FREQUENCY MODULATIONS, REPRESENTATIVE OF A-B, TO PRODUCE A SECOND RADIO FREQUENCY SIGNAL HAVING ENVELOPE AMPLITUDES CORRESPONDING TO SIGNAL B, SAID FIRST CHANNEL INCLUDING FIRST DETECTOR MEANS FOR DEMODUALTING SAID FIRST RADIO FREQUENCY SIGNAL, AND SAID SECOND CHANNEL INCLUDING SECOND DETECTOR MEANS TO DEMODULATE SAID SECOND RADIO FREQUENCY SIGNAL. 